A sensor having an EIS structure includes a semiconductor substrate on which an insulator is arranged, which is supplied with an electrolyte in measurement operation.
ISFETs are established examples of sensors having an EIS structure, wherein, in this case, the insulator forms the ion-sensitive gate insulator of a field effect transistor.
In the case of the so-called LAPS (light addressable potentiometric sensors), by means of a modulated light signal, photoelectrons are produced in the semiconductor material—an EIS structure—wherein the generating of photoelectrons, in turn, depends on the electrolyte properties. A basic description of LAPS is given by Hafeman et al. in “Light Addressable Potentiometric Sensor for Biochemical Systems”, Science 240 (1988) 1182-1185.
ISFETs are established and better investigated than other EIS structures. Therefore, in the following description of problems in the state of the art, reference is essentially made to ISFETs, in which case it is understood that these problems exist correspondingly for other sensors with an EIS structure.
Ion-sensitive field effect transistors ISFET) are applied for measuring ion concentrations or special substance concentrations in solutions of various compositions and conductivities. Applications of ISFETs for continuous detection of concentrations are to be found in environmental monitoring, in industrial process monitoring, in the foods industry and in biochemistry/medical technology. In such cases, a highly precise registration of concentration, fast start-up and minimal long-term drift of the sensor are especially of importance, coupled with an acceptable price.